AW: [PATCH v3 5/6] md/raid5: activate raid6 rmw feature

[Markus Stockhausen <stockhausen@collogia.de>]

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> Von: John Stoffel [john@stoffel.org]
> Gesendet: Sonntag, 24. August 2014 14:46
> An: Markus Stockhausen
> Cc: linux-raid@vger.kernel.org
> Betreff: Re: [PATCH v3 5/6] md/raid5: activate raid6 rmw feature
> ...
> Also, how does the patch look when you have a simple 4 disk RAID6 array?  
> I would hope that since the parity overhead is much higher, it would show 
> more improvement as well.

Maybe I did not explain exactly what this patch is for. To boil it down to four
short and simple key facts.

- The current kernel implementation of RAID6 is very I/O expensive for small 
changes. We always need to read all unchanged blocks of the same stripe, 
recalulate the parity and write the new data block plus two parities. So a single
block change will produce n (number of disks) I/Os. The source code calls it 
RCU - reconstruct write.

- With the patch we will only read the old data of the to be changed block & the 
parities, recalculate the parities with the new data and rewrite everthing back. A 
single block change will only have 6 I/Os regardless of the number of disks. It 
is called RMW - read modify write. 

- This patch will only kick in if you have 7 or more disks in a RAID6. And ONLY
if I/Os RMW < I/Os RCU. This should answer your question above.

- The more disks we have and the smaller the changed data is the more benefit
we will get from that patch.  

Markus=

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